How many cores are supported in PS as of today? The last time I read up on this (CS6), Photoshop would use only 6 cores, for the most part. What's the situation today - how many cores has PS CC 2017 been written to take advantage of?

I spoke with a couple of people about this topic today. While there are some limited factors and caveats, Photoshop will probably handle as many cores as are available. The question is actually moot with regard to any filters which make use the GPU since they can benefit from those resources in lieu of the cores. And many of the previous bottlenecks have similarly been addressed and/or removed.